Copper (Cu) interconnect technology is one of the most critical technologies in modern CMOS integrated circuits. However, copper is readily diffused into silicon and other dielectric materials. This will result in the failure of an integrated circuit if no effective isolation is imposed. It is reported that metal nitrides (such as TaxNy, TixNy and WxNy) take advantage of not only favorable copper anti-diffusion performance, but also good conductance and adhesion with low k dielectrics like SiOC, which make them preferred diffusion barrier materials in a copper interconnect process. In addition, these metal nitrides are also applicable for gate materials in MOSFET, which offers an optimal way for the integration between a high k dielectric and a metal gate. Furthermore, metal nitrides can also serve as electrode materials of metal-insulator-metal capacitors in RF integrated circuits.
Traditionally, the methods for preparing metal nitrides include physical vapor deposition (PVD) and chemical vapor deposition (CVD). Nevertheless, with the constant scale down of critical size, enlargement of device integration density, and shrinking of the distance between devices in integrated circuits, three dimensional structures have become the mainstream and the aspect ratio of vias in backend interconnect process is continuously increased. All these changes pose rigorous challenges such that traditional PVD and CVD technologies fail to meet the requirements of integrated circuits development, i.e., achieve the growth of ultrathin metal nitride films, the filling of high aspect ratio trenches, the formation of highly conformal coverage, and favorable uniformity for a large area. For example, 100% step coverage can be achieved for CVD only when the trench aspect ratio is below 10:1 and only 50% step coverage can be obtained for PVD under the same conditions. Additionally, it is difficult to ensure the growth of ultrathin metal nitride film and the uniformity of a large area for a CVD process. In contrast, ALD can achieve the above requirements. For example, metal nitride films can be deposited by the reaction of a metal-organic precursor with NH3 plasma. However, based on the ALD growth theory, metal and nitrogen atoms are alternatively produced, i.e., metal and nitrogen atom ratio is stable in the film. As a consequence, the relative metal and nitrogen atom ratio cannot be modulated and especially the relative percentage content of metal atoms cannot be enhanced, which restrains the achievement of metal nitrides with lower resistivity.
From the above description, a new kind of manufacturing process is essential to be developed for the preparation of metal nitrides with adjustable metal contents.